The present invention relates to a photodetector for use in a remote-control apparatus employing near infrared rays, etc. as control signals, for example, a remote-control unit of a television set, a video tape recorder or the like.
Conventionally, a reception portion of a wireless remote-control system based on near infrared rays is constituted by a photosensor (photoelectric conversion device), an amplification circuit, a detection circuit, a waveform shaping circuit, etc. This reception portion handles extremely weak electric signals and therefore, is readily affected by noises produced by electrostatic induction or electromagnetic induction inside and outside the wireless remote-control system, so that operation of the reception portion becomes unstable. Thus, the reception portion is enclosed by a shield casing such that entry of noises into the reception portion is restricted by the shield casing.
However, in order to receive electric signals by the photosensor, a window for receiving light is required to be provided on the shield casing. Hence, in order to prevent entry of noises into the reception portion through the window, various countermeasures shown in FIGS. 1 to 5 have been taken so far. In FIG. 1, a light receiving window 3 is formed as small as possible on a front face of a shield casing 2 of a reception portion 1. In FIG. 2, the light receiving window 3 is constituted by a conductive metal gauze. In FIG. 3, the light receiving window 3 is constituted by a plurality of slits as disclosed in Japanese Utility Model Publication (examined) No. 23182/1987. Meanwhile, in FIGS. 4 and 5, the light receiving window 3 on the front face of the shield casing 2 is constituted by a plurality of holes 4.
However, in the known arrangement of FIG. 1, since the diameter of the light receiving window 3 cannot be made so small due to restriction of directivity characteristics of the reception portion 1, the shield casing 2 cannot shield the reception portion 1 fully from noises. In the known arrangement of FIG. 2, directivity characteristics are inferior. Furthermore, in the known arrangement of FIG. 2, since the light receiving window 3 is constituted by the metal gauze separate from the shield casing 2, its production cost becomes high. Meanwhile, in the known arrangement of Fig. 3, since the slits extend longitudinally in the vertical direction of the shield casing 2 and opposite sidewalls of each of the slits are directed towards the photosensor, directivity characteristics for light signals proceeding in the lateral direction of the shield casing 2 are excellent but directivity characteristics for light signals proceeding in the vertical direction of the shield casing 2 become poor. Furthermore, in the known arrangement of FIGS. 4 and 5, since areas of the holes effective against light signals 5 proceeding obliquely relative to the front face of the shield casing 2 are substantially reduced as shown in Fig. 5,the amount of the light signals 5 received by a photosensor 6 is reduced, thereby resulting in deterioration of its directivity characteristics.